Pesticides are chemicals designed to cause biologic harm. Widespread use in agriculture and in urban areas creates an opportunity for exposure to a large segment of the population. Epidemiologic investigations are undertaken to identify and clarify cancer risks from pesticide exposure. During the past year a cohort study of lawn care workers was completed. Mortality rates for this cohort were similar to the general U.S. population for all cancers except non-Hodgkin's lymphoma where a small excess occurred among individuals engaged in this work for several years. Case-control studies of non-Hodgkin's lymphoma noted an association with agricultural use of lindane, but not with DDT. Methodologic studies are designed to improve research technics to conduct such investigations. A recent study in Iowa noted that farmers can recall considerable detail on pesticide application and that probing by the interviewer improved recall. Several feasibility studies have been conducted among migrant and seasonal farm workers to determine appropriate methods for epidemiologic investigations in this under studied group. Several projects to evaluate the cancer risk from pesticide exposure are underway. The Agricultural Health Study is a major effort designed to evaluate the relationship between pesticides and other agricultural factors and the risk of cancer and other diseases. Enrollment in this study includes about 90,000 men and women from Iowa and North Carolina. Detailed information is being obtained by interview on pesticides and other agricultural exposures, lifestyle factors, medical and family history of disease, and diet. The cohort will be linked to the National Death Index to obtain information on cause of death and to cancer registries to obtain information on cancer incidence. Recently completed case-control studies of cancers of the brain and stomach in Nebraska will provide the opportunity to assess the role of agricultural, general environmental, and lifestyle factors in the development of these tumors. Blood and urine have been collected from 30 applicators applying the herbicide 2,4-D to relate dose to possible biologic effects including DNA mutations, chromosome aberrations, and immune system alterations.